(1) Among thousands of clones selected from cDNA libraries of differentiation-regulated mRNAs from NS20Y neuroblastoma cells, we have selected three clones which are differentiation- specific. All of these are induced by growing the cells on a treated polystyrene surface, one is further stimulated by the addition of dibutyryl-cyclic AMP to the medium, two of them hybridize to transcripts in brain RNA, and two hybridize to a smaller size transcript from glioma cells and to a large and very abundant RNA in a human hepatoma cell line. (2) Several cDNAs for mRNAs which are unique to, or enriched in, the sub-chronic reaction around a stab wound in rat cerebral cortex have been isolated and are being evaluated for their relationship to the wounded brain neurotrophic factor. (3) Three cDNA clones from a rat hypothalamus library have been selected using synthetic oligonucleotide probes designed to detect the hydra head-forming peptide, a completely conserved dodecamer found in high concentrations in nervous tissues from coelenterates to man. These three clones have inserts of the same size, suggesting that they may represent full-length complements of the mRNA. (4) RNA preparations from cultured mouse glial cells have been injected into Xenopus laevis oocytes, and the translation products of these oocytes were active in stimulating the activity of the enzyme choline acetyltransferase in primary cultures of mouse fetal spinal cord or septum cells. Hence we have an assay for the enrichment of the factor encoding mRNA and for the screening of clones via hybrid selection methods.